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e46 Differential Q's and A's

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I found this handy info on e46fanatics.com

What is a differential? Do I gain horsepower by swapping it?
A differential is what distributes power to your drive-wheels from the engine/tranny, via a driveshaft. You don't gain any horsepower from swapping to a higher ratio, but you do change when the power is put down. Through torque multiplication, it feels like you have more power, when in reality, you're just changing when you put it down. So it feels like you gained torque, but you're just getting the power more towards the low end. Read this if you want a good description.

Well sweet gee willickers Ken! I have an E46, but I is too fail to find what diff ratio I have. Can I findz outs?
Yes, you can findz out.

330 Non-ZHP
Steptronic: 3.38
Manual: 2.93 (3.07 convertible)

330 ZHP
Steptronic: 3.64
Manual: 3.07

325
Steptronic: 3.46
Manual: 3.15 (3.46 convertible/touring)

328
Steptronic: 3.46
Manual: 2.93

323
Steptronic: 3.46
Manual: 3.07 (3.46 convertible/touring)

M3
Manual: 3.62 LSD


Cool beans Kenneth! Now...are all differentials between E46s interchangable? What other BMWs can I pull a diff out of and plop into my car?
Why, excellent question young man! All E46 diffs are interchangable except for the M3 diff, which will require that you swap the rear subframe to even bolt up. You can also pull a differential out of a Z4 or an X3.


Yay! Knowledge! Now, where can I find a differential? I looked it up at the dealer, and gosh! They are expensive! What can I do?
Do not buy it new. You have two real options, A) look on eBay and hope for the best, or B) go onto http://www.car-part.com and do a search of junkyards in the U.S. You will have to call them and order it (most junkyards have very...unfriendly employees, but it's worth it). You can usually find a diff anywhere from $200-$450. Any more and you're wasting your money/time.


Ken, I have a manual transmission, I want more peppyness, what are most people swapping into their cars?
3.38's and 3.46's are very popular to swap into manuals because it yields a difference, and doesn't destroy your gas mileage too badly. You can pull a 3.38 out of a steptronic 330, whereas the more widely available 3.46 (therefore more popular) can come out of a steptronic 325, steptronic 323, steptronic 328, steptronic 3.0 Z4, or manual 2.5 Z4.


Well poopsticks, what if I have a steptronic? What can I swap in that will make a difference since I already come with a 3.38 or 3.46?
You have two real options to feel any sort of gain on an auto, and it is definitely one of the best things you can do to get some more pep out of your slushbox. One would be to pull the 3.64 out of the steptronic ZHP, steptronic Z4, or steptronic 3.0 X3. Another would be to use the 3.91 out of the 2.5 X3. I did the 3.91 swap, and while it does take some work, it is worth it. Here is another thread covering that.


Oh noes, I want both my rear tires to do the spinny dance. What if I want an LSD on my non-M? What can I do?
Either you spend a ton of money to get the M3 rear subframe and fit the 3.62 out of the M into your car, or you spend a ton of money on a custom built diff from Quiafe or Diffsonline. Either way, you're going to spend a ton of money.


This is all fine and dandy, but what about installation?
A typical shop will charge you anywhere from $150-$300 to install it. You can do it yourself with a buddy if you feel mechanically inclined. Here is a DIY.


What kind of stinky poo smell fluid do I need to use?
Synthetic 75w-90 gear oil. I recommend Redline, you can get it from Turner Motorsport. You will need 2 bottles, and new seal rings (BMW part# 07119963355). You can find a DIY here.


Just posting for anyone interested and for myself for future reference.

If you want to DIY theres one here:
http://www.bokchoys.com/differential/diy_diff.htm

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